1. Field of the Invention
The present invention relates generally to optical devices, and more particularly to techniques for reducing or controlling the polarization sensitivity of such devices.
2. Description of Related Art
Computer and communication systems place an ever-increasing demand upon communication link bandwidths. It is generally known that optical fibers offer a much higher bandwidth than conventional coaxial links. In conventional optical communication systems, the optical power output from a light source is intensity modulated (IM) to generate digital signals carried by the optical fiber.
Phase modulated (PM) systems can provide higher data rates and longer-distance transmission than IM based systems. In PM optical communication systems, the digital signals are generated by modifying the phase of the light. For example, optical differential phase-shift keying (DPSK) is an optical signal format in which the digital information is encoded as the phase difference between adjacent bits. In differential quadrature phase-shift keying (DQPSK) the digital information is encoded using a constellation of four points, equispaced around a circle. With four phases, DQPSK can encode two bits per symbol. Although DPSK and DQPSK can achieve higher data rates and longer-distance transmission, these signal formats can require a relatively complex receiver. Monolithic DPSK or DQPSK receivers are described for example in International Publication No. WO 2009/038662 to Doerr, incorporated herein by reference.
The differential phase between two adjacent bits can be detected in a DPSK receiver chip using a Mach-Zehnder Delay Interferometer (MZDI) in conjunction with two photo-detectors. A typical MZDI includes unequal-length delay lines having a path length difference designed to be approximately one signal length of the data signal. (An MZDI is a special case of a Mach-Zehnder Interferometer (MZI), which can have equal or unequal length delay lines.) However, the delay in an MZDI is typically polarization dependent. This dependency deteriorates the accuracy by which the bits can be reconstructed by the receiver. A typical industry requirement on the polarization dependent accuracy for the phase difference in the delay of an MZDI is 3 degrees. For a DPSK data signal of around 40 Gb/s, this translates into a polarization dependent frequency shift of 0.3 GHz, which is extremely challenging in conventional MZDIs due to process variations in the current state of the art fabrication facilities. Currently, the standard deviation for the polarization dependent frequency shift in manufactured MZDIs is about 2 GHz.